Borosilicate glasses and second surface mirrors thereof

ABSTRACT

Radiation stable glass for space applications, especially glass in the form of thin sheet for cladding of spacecraft, is produced by including at least 5% by weight of barium oxide in a borosilicate glass composition. Because barium has a low absorption in the ultra violet, its use enables radiation stable glasses of low ultra violet absorption to be produced, alleviating problems of overheating when the glasses are used for cladding space craft. The glass is useful in the production of solar cell cover slips and second surface mirrors for cladding purposes, and space applications generally.

This application is a divisional of application Ser. No. 08/760,995,filed Dec. 5, 1996 now U.S. Pat. No. 5,895,719.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to glasses having good stability to radiationuseful as cladding glasses in space and terrestrial applications, and tocladding panes composed of such glasses.

On irradiation with high energy radiation typically encountered inspace, glass tends to discolour, reducing the transmission of the glassand increasing its solar absorptance. Thus, radiation stability is aparticular requirement of glasses used as cladding glasses in spaceapplications, for example, as solar cell cover slips or as the glasssubstrates of second surface mirrors used as cladding to protectspacecraft from overheating.

2. Description of the Prior Art

It is known, for example from EP 0261 885A1 and EP 0 505 061A2, to useborosilicate glasses, stabilised against the effects of irradiation bythe incorporation of cerium (typically in amounts of 2% to 5% byweight), for production of solar cell cover slips having a hightransmission in the visible and infra-red regions of the spectrum.Cerium has very broad absorption bands in the ultra-violet region of thespectrum at 240 nm and 315 nm. This absorption in the ultra-violet maybe beneficial when the glasses are to be used in solar cell cover slips,for example, in protecting the adhesive used to bond the cover slips tothe cells from ultra-violet radiation which would otherwise tend todegrade the adhesive. However, when the same base glasses are used, witha reflective coating on the back surface, as second surface mirrors toclad the exterior surface of a space craft and reflect unwanted solarradiation incident upon it, the absorption in the ultra-violet leads toan undesirable build-up of heat in the glass.

Thus there is a need for a method of stabilising a high transmissionborosilicate glass to radiation especially radiation encountered inspace, which does not rely on the use of cerium (or any other elementwhich absorbs significantly in the spectral region from 250 nmwavelength to 2500 nm wavelength). It has now been found, and thediscovery forms the basis of the present invention, that borosilicateglasses may be stabilised against radiation by inclusion of barium.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a radiation stableborosilicate glass in sheet form having a thickness less than 1 mmcharacterised in that the glass contains more than 5% by weight ofbarium (calculated as barium oxide) whereby its radiation stability isenhanced.

Preferably, the glass sheet has a cut-on of less than 340 nm (i.e. thetransmission of the glass sheet increases above 50% at a wavelength lessthan 340 nm).

The radiation stability of a glass in space may be estimated bysubjecting a thin polished sample of glass to an electronic bombardmentin vacuum and measuring the change in the optical characteristics of theglass. Radiation stable glasses typically have a radiation stabilitysuch that, if a polished sample pane of the glass 150 microns thick isexposed to 5.7×10¹⁵ 1 MeV electrons per square centimeter of glass invacuum (<1×10⁻³ torr), its solar absorptance changes by less than 0.05.Solar absorptance is the ratio of radiant energy absorbed by a body tothat incident upon it in the region 250 nm to 2500 nm integrated overthe air mass zero solar spectrum. Preferred glasses show a change insolar absorptance of less than 0.04, and especially preferred glasseschanges of less than 0.03. For use in second surface mirrors for thecladding of space craft, it is preferred to select glasses having asolar absorptance, after testing as described above, of less than 0.06,and preferably less than 0.04.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE of the drawings illustrates, in diagrammatic form, asection through a second surface mirror in accordance with theinvention.

The second surface mirror 1 comprises a thin (less than 1 mm, typicallyaround 0.2 mm) pane 2 of borosilicate glass with a reflecting coating 3,for example of silver, on the back surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The barium serves not only to stabilise the glass against radiation, butaids melting of the glass. Its presence also enhances the emissivity ofthe glass which is important for a cladding glass which is required tofacilitate heat loss through the glass. However, while barium ispreferably present in an amount of over 8% (calculated as barium oxide)and may be present in an amount up to 25% (calculated as barium oxide),amounts above 20% will normally be avoided for space applications (wherethe contribution of barium to the weight of the glass becomessignificant), and preferred glasses contain 8 to 18% of barium oxide.

Silica is the principal network former in the glass composition. Inamounts of 50 to 75% by weight it provides good durability to chemicalattack. Preferred glasses contain up to 70% by weight SiO₂.

The use of boron oxide is believed to assist the radiation stability ofthe glass and to provide a glass of high emissivity; it is also a usefulflux for glass making. At least 5%, and preferably at least 10% B₂O₃will normally be present in the glass. Unfortunately, excessive boroncontents tend to detract from the durability of the glass and the B₂O₃will therefore not normally exceed 30% by weight, with a B₂O₃ content inthe range 10 to 20% by weight being preferred.

Alumina assists in improving the durability of glasses without adverselyaffecting its solar radiation stability, and also assists in achievinghigh emissivity. When alumina is used in the glasses of the presentinvention, it will normally be present in an amount up to 15% by weight,and such that B₂O₃+SiO₂+AlO₃ is in the range of 60 to 93% by weight.

Zirconia plays a similar role to alumina, providing durability withoutadversely affecting the radiation stability. If used, zirconia willgenerally be present in an amount up to 12% by weight, but preferablyless than 5% by weight.

The alkali metal oxides are useful as fluxes for melting the glass,although when used in too high a proportion the durability of the glassmay be adversely affected. Thus the total alkali metal oxide content ofthe glasses of the present invention will normally be maintained below25% by weight, with alkali metal oxide content preferably in the range 8to 20% by weight. Potassium oxide is favoured since it is believed toassist in stabilising the glass against radiation damage; moreover,despite its high atomic weight, its inclusion tends to lead to a lessdense structure. The inclusion of potassium also tends to increase theelectrical conductivity of the glass, helping to prevent build-up ofstatic electricity. Sodium oxide may also be used as a flux, theadvantage being a more electrically conductive glass, but usually withless radiation stability. To maintain high radiation stability, sodiumoxide will normally be restricted to a maximum of 10% by weight whenused in the glasses of the present invention. Lithium oxide greatlyassists in melting the glasses even when present in small amounts. Above2% by weight lithium oxide, the risk of phase separation anddevitrification increases significantly and it is preferred to avoidmore than 3% lithium oxide.

Thus, according to a preferred aspect of the present invention, the thinborosilicate glass sheet of the invention comprises, in percent byweight:

5 to 20% of BaO

5 to 30% of B₂O₃

50 to 75% of SiO₂

0 to 15% of Al₂O₃

provided that B₂O₃+SiO₂+Al₂O₃ is in the range 60% to 93% and 2 to 25% ofR₂O where R₂O is Li₂O and/or Na₂O and/or K₂O. Other materials used inglass making may be included provided they do not have any unacceptabledetrimental effect on the glass. Thus, in addition to barium oxide,other alkaline earth metal oxides especially magnesium oxide, calciumoxide and strontium oxide may be present and will aid the melting of theglass. However, a total alkaline earth metal oxide content above 25%will normally be avoided, as it leads to a deterioration in glassdurability and can adversely affect the stability of the glass toradiation.

Refining agents, such an antimony oxide, Sb₂O₃ and arsenic oxide, AS₂O₃,which are commonly used in amounts of around 0.5% by weight, may beused, but will not generally be present in amounts of more than about 2%by weight. If required, for example to aid refining, the viscosity ofthe glass may be reduced by incorporation of a small amount of fluorine,typically up to about 1% by weight.

The presence of rare metal and other heavy metal non-colouring metaloxides may be tolerated in small amounts, for example up to about 5% byweight, but they are preferably avoided for space applications wheretheir weight would be a penalty. Zinc oxide and lead oxide arepreferably avoided (and, if present, each used in amount of less than 2%by weight) as their presence tends to lead to darkening of the glass onirradiation. Tin oxide may be used, and appears to decrease the solarabsorptance of the glass.

The radiation stability of the glasses may be improved still further bythe inclusion of cerium oxide and/or titanium oxide. However, as notedabove, cerium absorbs strongly in the ultra violet, while titanium hasweak absorptions in the ultra violet region of the spectrum. Thus, wherea low solar absorptance is important, as for second surface mirrors forcladding space craft, only small amounts of cerium (say up to 2% byweight, calculated as CeO₂), or titanium (say up to 0.2% by weight,calculated as TiO₂) will normally be used, and carefully controlled toensure that a satisfactorily low solar absorptance is achieved. However,for applications where a greater increase in absorptance in the ultraviolet region is acceptable, greater amounts of ceria (say up to 7% byweight) and titania (say up to 1% by weight) may be acceptable.

Colouring metal oxides will generally be avoided where possible, sincetheir presence will reduce the transmission of the glass (increasing itssolar absorptance), although small quantities, for example arising fromimpurities, may be tolerated provided their use does not result in anyunacceptable detrimental effect on the radiation stability of the glass.

The barium containing glasses used in the thin glass sheet of thepresent invention have surprisingly good radiation stability and areuseful for other purposes. Many of the glass compositions are novel and,according to a further aspect of the present invention, there isprovided a radiation stable borosilicate glass characterised in that itcomprises, in percent by weight:

<8% of BaO

6 to 30% of B₂O₃

50 to 75% of SiO₂

0 to 15% of Al₂O₃

provided that B₂O₃+SiO₂+Al₂O₃ is in the range 60% to 93% and 7 to 25% ofR₂O where R₂O is Li₂O and/or Na₂O and/or K₂O.

The invention is illustrated but not limited by the exemplary glasscompositions (suitable for the production of thin sheet for use insecond surface mirrors and solar cell cover slips) set out in thefollowing Table 1. In Table 1 the glass compositions are shown inpercentages by weight calculated on an oxide basis, assuming theelements to be present in the form of the oxide shown (except thatfluorine is calculated as such).

The glasses of Table 1 were melted in an electric furnace (althoughfossil fuel furnaces may be used if required) at a temperature of 1400°C. Their forming temperatures (i.e. the temperatures at which theyexhibited a viscosity of 10,000 poise, referred to in the table as “log4”) were within normal ranges for drawing into thin sheet glass(“microsheet”) of thickness less than 1 mm by, for example, known downdraw processes. Their liquidus temperatures were lower than the “log 4”temperatures indicating that may be formed into sheet withoutdevitrification faults. The measured log 2½ values (i.e. thetemperatures at which the glasses exhibit a viscosity of about 300poise) indicate that the glasses can be melted at normal glass makingtemperatures.

The chemical durability values quoted were determined using the ISO 719standard test. In the test, 0.2 grams of glass grains are boiled with 50ml of grade 2 water for 60 minutes at 98° C. and the released alkalititrated against 0.01M hydrochloric acid. For applications in accordancewith the present invention, a durability of at least HGB3, preferablyHGB2 or HGB1, is desirable.

Table 2 shows transmission properties of the glasses (measured over apath length of 150 microns) before and after irradiation of a glasssample 150 microns thick with 1 MeV electrons at a fluence of 5.7×10¹⁵e/cm². The “cut-on” is the wavelength at which the glass exhibited a UVtransmission of 50%, UVT is the percentage transmission over the range300 to 320 nm, while T400, T500 and T600 are the percentagetransmissions at wavelengths of 400 nm, 500 nm and 600 nm respectively.

The corresponding “target” figures for a glass for use in second surfacemirrors, together with the measured figures for a number of knownglasses, are shown in the early columns of Table 2. Preferred glassesfor this application achieve corresponding values after irradiationunder the conditions indicated above of <340 nm (“cut on”), >86%(T400), >88% (T500) and >90% (T600), whilst the best glasses exhibit<340 nm (“cut-on”), >88% (T400), >90% (T500), >91% (T600) with a changein solar absorptance on irradiation (as described above) of less than0.03.

TABLE 1 Example 1 2 3 4 5 6 7 8 SiO₂ 55 50 70 62 70 51 59.5 59.2 Li₂O 2Na₂O 4 2 K₂O 10 20 5 10 6 10 10 10 BaO 10 10 15 10 6 10 10 10 B₂O₃ 15 1510 10 6 15 15 15 Al₂O₃ 10 5 5 5 5 5 Sb₂O₃ TiO₂ 1.0 CeO₂ 0.5 0.8 OtherSnO₂ ZrO₂ 3 12 Liquidus temp no no (850-1150° C.) crystals crystals seenseen Durability HGB1 HGB2 log 2½ 0° C. 1460 1365 log 4 0° C. 1115 11761072 Strain point 556 575 0° C. Annealing 585 601 temp 0° C. Exp 65.765.7 (20-300° C.) Electrical conductivity 0° C. 10^(−17.3) 10^(−18.6)60° C. 10^(−14.8) 10^(−14.8) Density gram/ 2.4950 2.53 cc Example 9 1011 12 13 14 15 SiO₂ 59 52.2 70.8 54.5 59.5 59.1 59.3 Li₂O 2 Na₂O 4 10K₂O 10 15 6 10 10 10 BaO 10 10 6 15 10 10 10 B₂O₃ 15 15 6 20 15 15 15Al₂O₃ 5 5 5 5 5 5 Sb₂O₃ 0.2 0.4 TiO₂ 0.1 0.1 CeO₂ 1.0 0.2 0.2 0.1 0.50.5 0.6 Other ZrO₂ F₂ 2.5 0.2 Liquidus temp No (850-1150° C.) crystalsseen Durability HGB2 HGB1 log 2½ 0° C. 1240 1195 log 4 0° C. 1008 950Strain point 0° C. 577 552 Annealing temp 601 572 0° C. Exp (20-300° C.)77.3 70.5 Electrical conductivity 0° C. 10⁻¹⁷ 10^(−14.3) 60° C.10^(−13.6) 10^(−10.4) Density gram/cc 2.627 2.6075

TABLE 2 Solar Cell Window Lead Barium Cover Glass¹ Pyrex² Flint³ Crown⁴Glass⁵ Target Cut on (nm) before <280 <280 294 <280 355 after <280 <280364 <280 360 <340 UVT before 91.4 90 81.0 89.2 0.1 after 68.2 67.4 34.971.5 0.1 T400 nm before 92.1 90.9 87.3 91.4 85.1 after 66.9 80.3 61.381.6 83.1 >83% T500 nm before 92.5 91.1 87.3 91.7 89.9 after 73.5 84.679.6 84.6 89.7 >86% T600 nm before 92.8 91.5 87.9 92.0 90.9 after 78.188.2 87.9 85.3 91.0 >87.0% Solar absorptance before 0.0560 after 0.0585<0.06 Change in solar 0.096 0.037 0.143 0.038 0.0025 <0.04 absorptanceEmissivity >0.8 Exam- ple 1 2 3 4 5 6 7 Cut on (nm) before <250 <250<250 289 325 after <200 <200 <200 282 293 331 UVT before 94.99 95.1892.2 90.6 83.8 87.4 30.10 after 87.54 90.84 82.8 64.0 74.0 80.3 21.32T400 nm before 91.47 91.16 92.6 91.4 91.6 89.2 91.32 after 86.37 88.4686.3 86.0 83.4 83.8 89.92 T500 nm before 91.78 91.42 92.5 91.6 91.7 89.991.75 after 89.62 89.34 88.3 90.2 88.5 87.0 91.38 T600 nm before 92.0391.48 92.7 91.8 91.8 90.4 91.80 after 89.55 89.46 89.2 91.4 89.9 87.591.65 Solar absorp- tance before 0.0013 0.0010 0.0082 0.0065 after0.0243 0.0217 0.0325 0.0239 Change 0.023 0.0207 0.0243 0.0174 in solarabsorp- tance Emis- 0.08932 0.8830 sivity Example 8 9 10 11 12 13 14 15Cut on (nm) before 335 333 294 324 289 328 326 328 after 341 337 320 328325 335 335 334 UVT before 7.23 10.98 57.68 43.0 57.9 23.68 29.16 22.76after 3.94 7.19 44.89 30.8 38.4 14.36 16.57 15.22 T400 nm before 91.0391.06 91.12 93.8 90.2 91.30 91.50 91.31 after 89.39 90.18 89.07 89.686.7 89.57 87.98 89.90 T500 nm before 91.81 91.81 91.50 94.0 90.9 91.6091.95 91.76 after 91.30 91.55 90.39 90.9 90.0 91.02 91.03 91.51 T600 nmbefore 91.99 91.96 91.64 94.0 91.4 91.75 92.08 91.98 after 91.62 91.8190.62 91.7 90.7 91.37 91.58 91.80 Solar ab- sorptance before 0.01430.0164 after 0.0323 0.0331 change in 0.0180 0.0167 solar ab- sorptanceEmissivity ¹Composition: 72.6% SiO₂, 13% Na₂O, 0.9% K₂O, 4% MgO, 8.4%CaO, 1.1% Al₂O₃ ²Composition: 80% SiO₂, 4.5% Na₂O, 0.3% K₂O, 0.1% CaO,12.5% B₂O₃, 2.6% Al₂O₃ ³Designation ELF 561452, composition 60% SiO₂, 4%Na₂O, 9% K₂O, 1% CaO, 26% PbO ⁴Designation DBC 564608, composition 49%SiO₂, 2% Na₂O, 5% K₂O, 30% BaO, 2% ZnO, 9% B₂O₃ and 3% Al₂O₃⁵commercially available borosilicate type glass, doped with nominal 5%cerium oxide.

Examples 1 to 6 show, in comparison with the prior art cerium freeglasses, the excellent stability of the barium borosilicate base glassto radiation. It is notable that the radiation stability is better thanthat of the known barium crown glass, presumably because of the lowerlevels of silica and boron in those glasses. The measured values ofsolar absorptance (Examples 3 to 6) after irradiation confirm that thehigh retained transmissions across the spectrum are leading to lowabsorption of solar radiation by the glass. Examples 4 to 6 show SnO₂,TiO₂ and ZrO₂ can be included in the base glass without deterioration ofradiation stability.

Examples 7 to 15 show that the radiation stability may be improved stillfurther by addition of cerium oxide, although, as the amount of ceriumis increased, the ultra violet transmission falls so that, above about2% CeO₂ the solar absorptance is likely to exceed the low values soughtfor second surface mirrors.

The measurements of chemical durability on Examples 1, 7, 10 and 13 showsatisfactory performances. It is believed that the inclusion of zirconiain Example 10 is helpful in maintaining the chemical durability in thepresence of a high proportion of potassium oxide.

The sample glasses all have a medium expansion coefficient of about 60to 80×10⁻⁷/° C. in the range 20 to 300° C. and are suitable for use ascover slips for silicon or gallium arsenide solar cells.

For weight reasons, the glasses having a density below 2.7 are preferredfor space applications.

The glasses of the present invention may be used in sheet form ascladding glasses for both terrestrial and space applications. Forterrestrial applications, the thickness of the glass will depend on therequired mechanical properties as well as the optical properties needed,but may be less than 1 mm. For space applications, weight is normallycritical, and the glass will commonly be used in the form of sheets orpanes less than 0.3 mm (300 microns) preferably less than 0.2 mm (200microns) thick. It will be appreciated that, when absorbing constituentsare used (such as cerium oxide and titanium oxide, which both absorb inthe ultra violet) the amount should be selected having regard not onlyto the required properties of the pane or sheet, but also to itsthickness. Thus, for example, the amount of cerium oxide that may beused, without increasing the solar absorptance to a value higher thanappropriate for a second surface mirror, will depend on the thickness ofthe pane used to form the mirror.

For second surface mirrors (also known as optical solar reflectors) theglass will normally be used in a thickness of less than 300 microns witha reflecting coating on a rear i.e. second surface of the glass. Suchsecond surface mirrors are used as passive thermal control devices onthe main bodies of satellites. They are used on the sun-facing sides ofsatellites and reflect incoming solar radiation while simultaneouslyradiating internally generated heat. Thus, for this application it ispreferred that the glass has not only a low solar absorptance (so a highproportion of incident solar radiation is reflected), but also a highemissivity, preferably at least 0.8 (to facilitate radiation ofinternally generated heat).

According to a further aspect of the present invention there is provideda second surface mirror comprising a pane of radiation stableborosilicate glass having a cut-on of less than 340 nm and a reflectingcoating on one surface thereof. A second surface mirror may comprise asheet of thin barium borosilicate glass according to the invention witha reflecting coating on one surface thereof.

The coating is preferably of silver, which may be overcoated with aprotective layer, for example a metal alloy. For this application, suchcoatings are normally deposited by vacuum techniques such as sputtering.

If desires, the second surface mirror may be provided with a conductivecoating, for example of tin doped indium oxide, on its front surface toalleviate build-up of static electricity on the surface of thesatellite.

According to a further aspect of the present invention, there isprovided a solar cell cover slip comprising a sheet of bariumborosilicate glass according to the invention.

We claim:
 1. A second surface mirror comprising a pane of radiationstable borosilicate glass comprising more than 5% by weight of BaO andhaving, over a path length of 150 microns, a cut-on of less than 340 nmand a reflecting coating on one surface thereof.
 2. A second surfacemirror according to claim 1 wherein the glass pane has a sheet thicknessof less than 1 mm and contains more than 5% by weight of barium,calculated as barium oxide, whereby its radiation stability is enhanced.3. A second surface mirror according to claim 1 wherein the glass paneis a glass sheet comprising, in percent by weight 5 to 20% of BaO 5 to30% of B₂O₃ 50 to 75% of SiO₂ 0 to 15% of Al₂O₃ provided thatB₂O₃+SiO₂+Al₂O₃ is in the range 60% to 93% and 2 to 25% of R₂O where R₂Ois Li₂O and/or Na₂O and/or K₂O.